To improve the impact properties of polypropylene homopolymers (and random copolymers), an elastomeric component is typically added, either by way of the production of an in-reactor blend of a propylene polymer and an elastomeric component (an impact propylene copolymer) or by way of compounding of a propylene polymer and an elastomeric component. In the former method the propylene polymer and the elastomeric component are produced in one or more reactors of the same process.
Both of these methods of improving the impact of polypropylene do not significantly contribute to increasing the melt strength of the resulting polypropylene impact copolymer.
And although both methods result in a propylene polymer with improved impact properties (when formed into articles), these impact properties often show an imbalance between the notched impact tested parallel to the polymer injection flow direction of the injection molded product, and the notched impact tested perpendicular to the polymer injection flow direction of the injection molded product. The notched impact tested on specimens cut perpendicular to the direction of orientation (that is perpendicular to the polymer injection flow direction), with the notch in the direction of orientation (parallel to the polymer injection flow direction), is typically much lower. The polymer injection flow direction is the direction or line along which the polymer is introduced into a mold or the direction or line along which the polymer is extruded. Polymers such as propylene polymers will tend to orient themselves along this line of direction.
What is desired is a polymer resin, which provides directionally balanced impact properties for articles made from the resin, while simultaneously having increased melt strength. Additionally, it would be desirable to provide a polymer resin, which is easy to fabricate into articles and exhibits a ductile-to-brittle transition temperature of 0° C. or lower.
As used herein, the following terms shall have the following meanings:
“Coupling agent” means a chemical compound that contains at least two reactive groups that are each capable of forming a carbene or nitrene group that are capable of inserting into the carbon hydrogen bonds of CH, CH2, or CH3 groups, both aliphatic and aromatic, of a polymer chain. The reactive groups together can couple polymer chains. It may be necessary to activate a coupling agent with heat, sonic energy, radiation or other chemical activating energy, for the coupling agent to be effective for coupling polymer chains. Examples of chemical compounds that contain a reactive group capable of forming a carbene group include, for example, diazo alkanes, geminally-substututed methylene groups, and metallocarbenes. Examples of chemical compounds that contain reactive groups capable of forming nitrene groups, include, but are not limited to, for example, phosphazene azides, sulfonyl azides, formyl azides, and azides.
“Impact propylene copolymers” are commercially available and are well known within the skill in the art, for instance, as described by E. P. Moore, Jr in Polypropylene Handbook, Hanser Publishers, 1996, page 220-221 and U.S. Pat. Nos. 3,893,989 and 4,113,802. The term “impact propylene copolymer” is used herein to refer to heterophasic propylene copolymers where polypropylene is the continuous phase and an elastomeric phase is dispersed therein. Those of skill in the art recognize that this elastomeric phase may also contain crystalline regions, which for purposes of the current invention are considered part of the elastomeric phase. The impact propylene copolymers result from an in-reactor process rather than physical blending. Usually the impact propylene copolymers are formed in a dual or multi-stage process, which optionally involves a single reactor with at least two process stages taking place therein, or optionally multiple reactors.
“Impact properties” refer to properties of articles such as impact strength, which is measured by any means within the skill in the art, for instance, Izod impact energy as measured in accordance with ASTM D 256, MTS Peak Impact Energy (dart impact) as measured in accordance with ASTM D 3763-93, and MTS total Impact Energy as measured in accordance with ASTM D-3763. The ductile-to-brittle transition temperature (DBTT) is also an impact property of an article made from a polymer. The ductile-to-brittle transition temperature defines, for a given set of conditions, the temperature at which an object transitions from a predominantly ductile mode of failure to a predominantly brittle mode of failure. The ductile-to-brittle transition temperature can be calculated using techniques known to one of skill in the art.